Current-limiting transformer



July 13, 1948. 1.. J. SCHILLING 2,445,038

' CURRENT LIMITING TRANSFORMER Filed Jan. 21, 1944 Egg 1.

I I V/////AV//////I! Patented July 13, 1948 2,445,088 CURRENT-LIMITINGTRANSFORMER Galcsville, Wis assignm- Lorcll John Schilling,

to Babson Bros. 00., a corporation of Illinois Application January 21,1944, Serial No. 519,129

3 Claims. 1

This invention relates to a current limiting transformer, and moreparticularly to a transformer having means associated with the shuntportion thereof to adjust the reluctance of such portion.

One feature of this invention is that it provides an improved currentlimiting transformer; another feature of this invention is that itprovides a current limiting transformer particularly adapted to supplyenergy to an electric fence; still another feature of this invention isthat, as

1 a commercial manufacturing proposition, the

maximum current output of the transformer may be closely and accuratelyadjusted to the desired value; and yet another feature of this inventionis that it enables current limiting transformers of the kind designed toenergize electric fences to be manufactured more quickly and cheaply,with fewer rejects, while at the same time limiting the output currentmore closely to the desired value. Other features and advantages of thisinvention will be apparent from the following specification and thedrawing, in which:

Figure 1 is a top plan view of a transformer embodying my invention;Figure 2 is a longitudihal section view along the line '2-2 of Figure 1:and Figure 3 is a transverse sectional view along the line 3-3 of Figure2.

Electric fences for livestock are steadily increasing in favor; and suchfences provide an electrical device which must be periodically energizedbriefly, with a high voltage, very low amperage electrical impulse..While some few electric fences are continuously energized with anexceedingly low maximum current delivered thereto, the type of fencewhich has proved most effective and which has become most popular isenergized about once each second for a very brief period. Experimentshave proved that an impulse of about three milli-coulombs (threemilliampere seconds) is very effective in stopping animals where theaverage voltage of the impulse is somewhere between 750 and 2,000 volts.Where the electric fence is energized from a conventional power supply(as 115 volt, 60 cycle current), it is desirable to limit the maximumcurrent deliverable by the transformer to 25 milliamperes; and this isrequired by the laws of several states. On the other hand, a transformerwhich has its output current limited to much below this value fails toprovide the desired "sting" necessary to stop animals and keep them frompushing through the electric fence, usually of only one or two wires.

While a transformer may be readily designed to deliver a desired outputvoltage (as for example 1500 volts) with a current limited to 25milliamperes, manufacturing such transformers on a production basiswithout substantial variations in the maximum current deliverable provesto present considerable difficulty. The cores of such transformersusually comprise a stack of stamped laminations of sheets of iron oriron alloys having the desired magnetic permeability characteristics.There may be 20 or 30 such laminations of generally rectangular formfour or five inches long and three or four inches wide, stacked up tomake a core in the neighborhood of an inch thick. The primary winding orcoil may have about 950 turns, and the secondary about 10,000 turns.With a transformer of these approximate dimensions, the central leg orshunt portion of the core should have an air gap therein provided byspacing the confronting lamination ends by about .020 inch, or slightlyless.

All sorts of variations are possible in such a transformer, and most ofthese effect the maximum current deliverable. The generally rectangularcore is generally formedby interleaving F-shaped laminations at oppositecorners, and the amount of interleaving cannot be accurately reproduced.The operation of stamping out the laminations sometimes creates smallburrs along certain of the edges, and these provide different spacingsand relationships between the laminations in building up the core. Thenumber of turns in the coils, particularly the secondary coil having inthe neighborhood of 10,000 turns thereon, cannot be controlled withabsolute precision without undue expense. All of these matters whichhave just been mentioned, and other factors entering into themanufacturing of such transformers, provide variations in the maximumcurrent deliverable. If the production transformers are to have anoutput never exceeding 25 milliamperes (and this must be the case tomeet the requirements of certain State laws), it has heretofore beenthought necessary to design the transformer to have a maximum output inthe neighborhood of 22 or 23 milliamperes,-so that there might be somevariation on the higher side during production without an undue numberof rejects. As a practical proposition, this resulted in a number oftransformers being produced which varied on the low side and limitedtheir current output to something around twenty milliamperes. This isundesirable for energizing an electric fence, since such a low maximumoutput, particularly if the interrupt-.

ing mechanism happens to be running fast and providing a still shorteroperative period than desired, does not deliver suiiicient "sting" toprovide an efficient electric fence.

I have devised a current limiting transformer, and a method ofmanufacturing such a transformer, which obviates these difficulties andenables electric fence energizing transformers to be manufactured, as aProduction proposition, with their maximum current outputs held towithin less than half of 1% variation. That is, an electric fencetransformer embodying my invention can be designed to deliver 24.9milliamperes with an open circuit secondary voltage of 1,500 volts; andsuch transformers can be manufactured with a production variation ofless than 1 6 of a milliampere in their output, so that the transformersnever deliver less than 24.8 milliamperes nor more than 25 milliamperes.I achieve this result by deliberately designing the transformers withconsiderably more reluctance in the shunt portion of the core (1. e., aconsiderably wider air gap) than would be necessary to effect thedesired output; and then by adjusting the reluctance of the shuntportion of the core to the desired value by bridging the gap with aselectable number of bridging laminations, precision final adjustmentbeing effected by movement of one of such laminations longitudinally ofthe shunt portion of the core.

Inasmuch as electric fences, their energizing apparatus, and currentlimiting transformers therefor, have been the subject .of previouspatents, only the transformer is illustrated here and the remainder ofthis specification will be limited thereto. If fuller information isdesired as to the electric fences and their energizing equipment andcircuits, reference may be had to issued patents, as for examplePfanstiehl Patents 2,249,696 (July 15, 1941) and 2,304,954 (Dec. 15,1942) previously issued to the assignee of the present application.

In the particular embodiment of my invention illustrated herewith, thetransformer is shown as comprising primary and secondary coils l and l Iencircling the legs lia and lib of a core li having a main flux circuitor path including the portions lia, lib, lie and lid and threading boththe primary and secondary coils. The core also has a central shuntportion lie comprising the two parts lie and lie" separated by the airgap l3 best seen in Figure 3. Whereas, in a transformer 01' the kindgiven as a representative example above, the air gap should be in theneighborhood of .020 inch to give the desired maximum current output,the air gap ii is made .025 inch or even wider. This enables substantialmanufacturing tolerances in the production and assembly of thetransformer while always ensuring that the fixed shunt portion of thecore will have a higher reluctance than that necessary to effect thedesired maximum output of 25 milliamperes.

The theory of operation of current limiting transformers of the kindshown here is well known, but it is felt that it would be helpful to anunderstanding of this invention if a brief description thereof is madeat this time. Magnetomotive force developed in the leg lia of the coreby current flowing in the primary l0 causes magnetic flux to travel themain flux path and thread the secondary coil H; and this results increation of an open circuit voltage across suchcoil II which bears thesame proportion to the primary voltage as the turns ratio. As anappreciable load is thrownacross the secondary coil. however, currentdelivered to such load by the coil creates a countermagneto-motive forcein the core leg lib which opposes the flux creating the voltage in thesecondary coil; and it will be apparent that the heavier the load (thelower the resistance across the terminals of the coil) the greater thiscountermagneto-force will be and the more it will oppose or buck passageof magnetic flux through the leg lib. Under normal conditions, the airgap in the shunt portion lie of the core makes this shunt path of suchhigh reluctance that substantially all ofthe flux passes through themain magnetic circuit including the leg lib. As countermagneto-motiveforce developed in the coil l l increases the effective or apparentreluctance of this leg, however, more and more flux passes through theshunt portion or shunt leg of the core. Since any flux passing throughthis shunt portion has no effect upon the secondary coil, variations inresistance across the secondary coil automaticall varies the amount offlux passing through that coil in such a way as to maintain the maximumcurrent deliverable at the desired value.

By making the air gap ll considerably wider than would be appropriate tothe maximum current desired, I am able to ensure the reluctance of theshunt portion always being greater than that necessary for the desiredmaximum current output. The reluctance interposed by the air gap maythen be reduced, and the effective reluctance of the entire shunt leg ofthe core thus reduced, by bridging the gap with a selectable number ofsmall bridging laminations here identified as lla-f. After thetransformer has been assembled, with the primary and secondary coils inplace and the corners of the F-shaped main laminations interleaved, themain core may be locked in place by bolting, clamping or any otherappropriate means. Input and output connections may then be made to thetransformer, the transformer energized with power of the same kind whichis to be delivered to it in operation, and the maximum output on shortcircuit noted on a sensitive milliammeter. A selected number of bridginglaminations may then be placed on the shunt portion of the core and thisnumber varied, by putting on or taking off such bridging laminations,until a maximum current very close to that desired is obtained. Finaladjustment of the maximum current output may be eflected by using ashorter bridging lamination, as the one here identified as Hf, andadjustably moving this longitudinally of the shunt leg to effect thefinal adjustment. That is, this smaller bridging lamination can becaused to extend only half way across the air gap, as shown; it may bepulled to the left, speaking with respect to the position shown inFigure 3, to reduce its effectiveness; or it may be moved to the rightto increase its bridging effectiveness and reduce the reluctance of theshunt path. After the adjustment has been effected, the bridginglaminations are locked in operative position on the shunt leg by anyappropriate clamping means which does not effect the magneticconditions. This is here shown as effected by a disk is of non-magneticmaterial, as Bakelite, here shown as operating not only as means to holdthe bridging laminations in place, but also as means for mounting thewhole transformer in a non-magnetic base plate ll, this being here shownas effected by the studs lid and I"). It will be understood that thebridgin laminations may extend the full width of the shunt leg,v

but preferably not its full length; and that the bridging means wouldhave a thickness only a very small fraction of that of the transformercore, as for example only five or six laminations, or even fewer, ascontrasted with about 30 laminations in the core. Under these conditionsthe bridging portion is very small compared to the fixed portions of theshunt leg of the core, and accidental loosening up or misadjustment ofthe bridging portion will not render the transformer output dangerous.

While I have shown and described certain embodiments of my invention, itis to be understood that it is capable of many modifications. Changes,therefore, in the construction and arrangement may be made withoutdeparting from the spirit and scope of the invention as disclosed in theappended claims.

I claim:

1..In a current limiting transformer having a ermeable core providing amain flux path and a shunt flux path across said main path, the shuntpath having an air gap therein, apparatus of the character described,including: primary and secondary windings on the main flux path of saidcore; bridging means of permeable material adapted to bridge said gapand afiect the reluctance of the shunt portion of the core, thisbridging means comprising a selectable number of laminations of saidpermeable material on the outside of the shunt portion; and means forlocking the bridging means in desired relation to said shunt portion. 2.Apparatus of the character claimed in claim 1, wherein at least one ofsaid laminations is adapted to be movably adjusted longitudinally ofsaid shunt portion of the core.

3. Apparatus of the character claimed in claim 1, wherein the lockingmeans comprises a nonmagnetic member adapted to overlie said bridgingmeans.

LOREIL JOHN SCHILLING.

REFERENCES CITED The following references are ofrecord in the file ofthis patent;

UNITED STATES PATENTS Number Name 7 Date 1,859,115 Summers May 1'1, 19321,863,936 Schwager June 21, 1932 FOREIGN PATENTS Number Country Date373,355 Great Britain May 26, 1932 606,024 France Feb. 27, 1926

